1. Field of Invention
The invention relates generally to microwave ovens and more particularly to resistance heating elements mounted therein and quarterwave chokes.
2. Brief Description of the Prior Art
Resistance heating elements in microwave ovens have commonly been of two types: a shielded rode heater or otherwise known as a "Calrod"; and quartz lamp heaters. In the former type, microwaves are shielded from the electrically conductive resistance element by action of the shielding element, which is commonly grounded to the microwave oven cavity wall. In the latter type of heating element, the resistance element has no shield. Therefore, unless shielded by other means, microwaves would impinge upon the conductor and be transmitted through the cavity wall. This is unacceptable. Thus quartz lamps, when included in microwave ovens, have commonly included metal shielding elements entirely enclosing the quartz lamp.
However, this structure has the drawback that the shielding structure substantially masks the transmission of infrared radiation. The performance of the quartz lamp heater is degraded. To overcome this limitation, a structure shown in Mittelsteadt et al, Ser. No. 434,301, employs a quarterwave choke structure mounted exterior to the microwave oven cavity about the exit point of the heater's power lead from a shielded housing. A similar structure is shown in Weiss U.S. Pat. No. 4,358,653 with respect to "Calrods.".
However, it has been found that the Mittlesteadt structure causes the power lead to heat up to an unacceptable degree at its point of exit from the microwave oven cavity. Similarly, the shielded housing structure itself heats to an unacceptable degree.